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THE PLACENTA                                   125

a special sucking mouth. This sucking mouth is an extra-uterine
production, and is of course an adaptation to the particular needs
of the young, just as are other larval organs, such as the chin-
suckers of the tadpole, or the regular ciliated bands of the
larvae of various marine invertebrate organisms.
There are a number of other features which distinguish the
Marsupials from other mammals.
The cloaca of the Marsupials is somewhat reduced, but is still
recognisable. Its margins in Tarsipes are even raised into a
wall, which projects from the body.
The tooth series of the Marsupials was once held to consist of
one dentition only, with the exception of the last premolar, which
has a forerunner. The interpretation of the teeth of Marsupials
are various. Perhaps most authorities regard the teeth as being
of the milk dentition, with the exception of course of the single
tooth that has an obvious forerunner. But there are some who
hold that the teeth are of the permanent dentition. In any case
it is proved that a set of rudimentary teeth are developed before
those which persist. Those who believe in the persisting milk
dentition describe these as prelacteal. Another matter of im-
portance about the teeth of this order of mammals is that their
numbers are sometimes in excess of the typical Eutherian 44.
This, however, holds good of the Polyprotodonts only,
It was for a long time held that the Marsupials differed
from all other mammals in having no allantoic placenta. But
quite recently this supposed difference has been proved to be
not universal by the discovery in jPerameles of a true allantoie
placenta. The Marsupials have been sometimes called the JDi-
delphia. This is on account of the fact that the uterus and the
vagina are double- Very frequently the two uteri fuse above,
and from the point of junction an unpaired descending passage
is formed (see Fig. 48 on p. 74).
A character of the brain of Marsupials has been the subject
of some controversy. Sir Richard Owen stated many years ago
that they were to be distinguished from the higher mammals by
the absence of the corpus callosum. Later still it was urged
that a true corpus callosum, though a small one, was present;
while, finally, Professor Symington1 seems to have shown that
1  **Th.e Cerebral Commissures in  tlie Marsmpialla and Monotretnata,"
Anat. Phys. xxvii. 1893, p. 69.